In order to be able to supply a continuously growing population with food and other plant-derived products, people have always been interested in improving the productivity in agriculture.
The productivity of a plant can be influenced in various different ways, e.g. by improving plant growth characteristics or by delaying leaf senescence. There are many mechanisms and pathways known which are involved in plant growth and development.
Cytokinin is a plant hormone that plays positive and negative regulatory roles in many aspects of plant growth and development. It stimulates the formation and activity of shoot meristems, is able to establish sink tissues, retard leaf senescence, inhibits root growth and branching, and plays a role in seed germination and stress responses (Mok, D. W. S. & Mok, M. C. (2001) Ann. Rev. Plant Physiol. Mol. Bio. 52, 89-1 18). Analysis of cytokinin-deficient plants has shown that cytokinin plays opposite roles in shoot and root meristems and suggests that the hormone has an essential function in quantitative control of organ growth (Werner T, Motyka V, Laucou V, Smets R, Van Onckelen H, Schmülling T, Plant Cell 2003, 15(11):2532-50; Werner T, Motyka V, Strnad M, Schmülling T, Proc Natl Acad Sci USA 2001, 98(18):10487-92).
It has been shown that cytokinin oxidases/dehydrogenases (CKX) are an important factor to regulate the homeostasis of the plant hormone cytokinin. The genome of Arabidopsis encodes seven CKX genes, which have distinct expression domains (Werner et al., 2001; Werner et al., 2003). The CKX proteins differ in their subcellular localization and biochemical features (Werner et al., 2003). Overexpression of individual CKX genes established cytokinin-deficient plants and revealed that cytokinin is a positive regulator of the shoot meristem activity and a negative regulator of root meristem activity.
Recently it was shown that in a rice plant inhibition of the function of a particular CKX gene, the rice orthologue to CKX3 of Arabidopsis thaliana, has led to an increase in particle-bearing number of said rice plant (see US 2006/0123507 A1).
Although these results are promising, there remains a need for further improving the productivity of plants.
It is an object of the present invention to provide means and methods suitable to improve productivity of plants.
This object is achieved by the present invention as set out in detail below.